The present invention relates to an improved recycle process for the preparation and processing of a hydroxylammonium salt solution, and in one embodiment thereof an apparatus used in carrying out the improved process. The hydroxylammonium salts are useful in the known preparation of oximes from ketones, in particular in the preparation of cyclohexanone oxime from cyclohexanone.
In a known recycle process for the production of hydroxylammonium salt solutions, an acidic, buffered, aqueous reaction medium containing buffer acids or acidic salts, for example phosphoric acid or bisulphate, respectively, and buffer salts derived from these acids, is continuously recycled between a hydroxylammonium salt synthesis zone, in which nitrate ions are catalytically reduced with molecular H.sub.2 to hydroxylamine, and an oximation zone. The nitrate ions, which are consumed in the hydroxylammonium salts synthesis zone, are added to the recycling aqueous reaction medium just before the hydroxylammonium salt synthesis zone. Nitric acid, such as a 60% aqueous solution of nitric acid, is commonly used as the source of nitrate ions. A hydroxylamine, initially formed in the hydroxylamine synthesis zone, reacts with the free buffer acid of the recycling medium to form the corresponding hydroxylammonium salt, and the resulting hydroxylammonium salt solution is then fed to the oximation zone, where the hydroxylammonium salt reacts with a ketone, forming the corresponding oxime, with simultaneous liberation of the acidic, buffered, aqueous reaction medium. The aqueous reaction medium is then freed of the oxime and recycled to the hydroxylammonium salt synthesis zone.
If the hydroxylamine preparation is based on the buffered reaction medium comprising phosphoric acid and nitrate, the chemical reactions occurring during the successive operations can be represented by the following equations:
1. formation of hydroxylammonium salt: EQU 2 H.sub.3 PO.sub.4 + NO.sub.3.sup.- + 3 H.sub.2 .fwdarw. NH.sub.3 OH.sup.+ + 2 H.sub.2 PO.sub.4.sup.- + 2 H.sub.2 O PA1 2. formation of cyclohexanone oxime: ##EQU1## 3. addition of HNO.sub.3 to replace the nitrate ions which were used in forming the oxime: EQU H.sub.3 PO.sub.4 + H.sub.2 PO.sub.4.sup.- + 3 H.sub.2 O + HNO.sub.3 .fwdarw. 2 H.sub.3 PO.sub.4 + NO.sub.3.sup.- + 3 H.sub.2 O
the buffered reaction medium preferably contains phosphoric acid.
The solution formed in step three will, theoretically, have the same composition as the initial solution used for the hydroxylammonium salt formation.
The catalyst used in the reduction of the nitrate ions is a palladium catalyst, generally palladium on a carrier material of carbon or alumina, the carrier material being loaded with from 5-20% wt. of palladium. The activity of the catalyst is adversely affected by the presence of organic substances. In the prior art process, the aqueous reaction medium which is discharged from the cyclohexanone oxime synthesis zone contains residual amounts of cyclohexanone and cyclohexanone oxime. The palladium catalyst is adversely affected by these residual amounts of cyclohexanone and cyclohexanone oxime and ultimately becomes poisoned. Accordingly, it is an object of the present invention to remove these residual amounts of cyclohexanone and cyclohexanone oxime to prevent poisoning of the catalyst.